1. Field of the Invention
The present invention generally relates to a load drive apparatus including a power transistor. More specifically, the present invention is directed to a protection circuit for a power transistor that is periodically turned ON/OFF in response to an oscillation signal produced by charging/discharging a capacitor.
2. Description of the Related Art
Conventionally, in either drive apparatus for turning ON/OFF lamps of flashing lights, or PWM control apparatus for driving motors and lamps in the PWM (pulse width modulation) signal, a power transistor is provided in series with a current path used to supply a DC current from a DC power source to a load. Then, this power transistor is periodically turned ON/OFF in response to an oscillation signal obtained by charging/discharging a capacitor.
In other words, in this sort of apparatus, the charging/discharging capacitor is charged/discharged via a resistor in such a manner that the voltage across this capacitor is varied between preselected two values. This periodical ON/OFF operation of he power transistor is performed in such a manner that the power transistor is turned ON during the charging operation of the capacitor, and turned OFF during the discharge operation thereof. Otherwise, such a drive signal (PWM signal) having a desired duty ratio is produced by comparing the voltages across the capacitor, which are varied by the charging/discharging operations, with the reference voltage, and then the power transistor is turned ON/OFF in response to this PWM signal.
On the other hand, normally such a protection apparatus as disclosed in Japanese Laid-open Patent Application No. 59-32224 opened in 1984 is employed in the power transistor used in this of load drive apparatus. That is, to protect the power transistor from overcurrents or overheat dissipation, which is caused by shortcircuiting, or disconnecting the current path into which this power transistor is inserted, this protection apparatus is operated as follows: After a predetermined delay time has passed since the power transistor is turned ON, a judgement is made whether or not overcurrents may flow through the power transistor by checking whether or not the voltage across the terminals of the power transistor connected to the current path of the load is higher than a predetermined voltage. It should be noted that this terminal voltage corresponds to a collector-to-emitter voltage VCE when a bipolar transistor employed as the power transistor, or this terminal voltage corresponds to a drain-to-source voltage VDS when a field-effect transistor (FET) is employed as this power transistor. When it is so judged that the overcurrents may flow through the power transistor, the power transistor is compulsorily and continuously turned OFF via a latch circuit constructed of a thyristor and transistors.
However, the above-described conventional protection apparatus own such a problem that heat dissipation of the power transistor is gradually increased and then the power transistor would be electrically destroyed even if the above-explained protection apparatus is employed. This is because when either the drive signal of the power transistor is brought into the OFF state, or the power supply switch is brought into the OFF state, which is separately provided in the current path from the power supply to the load, the latching operation by the latch circuit is released and the power transistor may be again turned ON if the subsequent load driving Operation is commenced, whereby when the power transistor is periodically turned ON/OFF by the above-explained drive apparatus, or the power supply switch is repeatedly turned ON, this power transistor would be overheated.
In other words, in accordance with the conventional protection apparatus, after a preselected delay time has passed since the power transistor is turned ON, a judgement is made of such an overcurrent caused by a shortcircuit of the load, or an increase in the internal resistance of the power transistor based upon the terminal voltage of the power transistor, and therefore the power transistor is turned OFF, whereby the power transistor is caused to be turned OFF. As a result, a current may necessarily flow through the power transistor for a predetermined time period after the power transistor is turned ON. Consequently, when either the power transistor is periodically turned ON/OFF by the above-explained drive apparatus, or the power supply switch is repeatedly turned ON/OFF, the current must flow through this power switch for a preselected time period even if the protection apparatus is employed. As a consequence, this current may emphasize heat dissipation of the power transistor, and finally could electrically destroy this power transistor.
Also, the conventional protection apparatus is susceptible to adverse influences of noise, and therefore owns such a problem that the power transistor is compulsorily turned OFF even when no overcurrent may flow through the power transistor. That is, for example, overcurrents are erroneously judged due to noise appearing in the current path where the power transistor is employed, so that the latch circuit would be operated, or the latch circuit would be directly actuated by this noise.
It should be noted that to prevent such an erroneous judgement of the over currents caused by the noise, after the overcurrents are judged from the terminal voltage of the power transistor, this judgement signal may be delayed by the integral circuit and the like to supply the delayed judgement signal to the latch circuit. In this case, such a delay circuit as the integral circuit should be separately provided, and nevertheless it could not prevent the erroneous operation of the latch circuit caused by the noise.